1. Field of the Invention
Aspects of the present invention relate to a desulfurizer for a fuel gas for a fuel cell and a desulfurization method using the same. More particularly, aspects of the present invention relate to a desulfurizer using absorbers that have selective absorption capacity for thiophene-based compounds and mercaptan-based compounds, and that includes multiple stages to perform a more efficient and economical desulfurizing of a fuel gas that contains various sulfur compounds, and a desulfurization method using the same.
2. Description of the Related Art
Fuel cells are electricity generation systems that directly convert the chemical energy of oxygen and the hydrogen contained in hydrocarbons such as methanol, ethanol, and natural gas to electrical energy.
Fuel cell systems consist of a fuel cell stack, a fuel processor (FP), a fuel tank, and a fuel pump. The fuel cell stack is the main body of the fuel cell, and comprises a plurality (several to several tens) of unit cells, each including a membrane electrode assembly (MEA) and a separator (or bipolar plate). The fuel pump supplies fuel from the fuel tank to the fuel processor. The fuel processor produces hydrogen by reforming and purifying the fuel and supplies the hydrogen to the fuel cell stack. The fuel cell stack receives the hydrogen and generates electrical energy by the electrochemical reaction of the hydrogen with oxygen.
Components of the fuel processor such as a reformer and a water-gas shift reactor, reform fuel gas for the fuel cell using a reforming catalyst and a shift catalyst and remove carbon monoxide. These catalysts, as well as the anode catalyst of the membrane electrode assembly, can be easily poisoned by the sulfur compounds.
FIG. 1 is a schematic graph illustrating the electrode voltage of a fuel cell against concentration of H2S, which is a sulfur compound. Referring to FIG. 1, poisoning of the electrode due to the sulfur compounds dramatically decreases the voltage of a unit cell as the density of the sulfur compound increases (operating condition: anode=0.4 mg/cm2 PtRu; cathode=0.4 mg/cm2 Pt; Nafion 112; Tcell=65° C.; P=1.5 bar; anode not humidified; cathode humidified at 65° C.; exposure to H2S starts at 24 hrs). For this reason, sulfur compounds need to be removed before performing a reforming process on fuel gas.
FIG. 2 is a block diagram conceptually illustrating the constitution of a conventional fuel processor for a fuel cell. As shown in FIG. 2, the conventional fuel processor includes a desulfurizer to perform a desulfurization process.
City gas (that is, gas from a municipal utility) is a potential source of fuel gas for a fuel cell. However, city gas typically contains about 10 ppm of sulfur compounds, which are deliberately added to the gas to function as odorants. The sulfur compounds added to city gas typically include a mercaptan-based compound such as tertiarybutylmercaptan (TBM) and an alkyl thiophene-based compound such as tetrahydrothiophene (THT), which are typically added in the ratio of 3:7. Accordingly, in order to use city gas in a fuel cell, the sulfur compounds need to be removed to obtain a concentration of 10 ppb or less.
Methods of removing sulfur compounds can employ a hydrodesulfurization (HDS) process or an adsorbent. Although the hydrodesulfurization process is reliable, it requires high temperatures of 300° C. to 400° C., is complicated to operate and thus is more suited to large-scale plants rather than small-scale devices. Accordingly, in order to remove sulfur compounds such as TBM, THT, etc., from the fuel gas used in a small-scale device, a method employing an absorbent, such as an absorbent made of activated carbon, metal oxide, zeolite, or the like through which a fuel gas passes, is more suitable. However, absorbents have the disadvantage of requiring replacement or regeneration when they become saturated with sulfur compounds. The amount of absorbent required and the cycle life of the absorbent are largely influenced by the absorptivity of the absorbent. Thus, an absorbent that has a high absorptivity is preferred.
Various absorbents have been suggested to remove sulfur in city gas. However, the suggestions have focused on absorbents that absorb a specific sulfur component. Adsorbents that selectively absorb each component of fuel gas containing two or more kinds of sulfur compounds is not yet known. For example, Japanese Patent Laid-Open Publication No. hei 1994-306377 discloses a zeolite for removing mercaptan from city gas, which is ion-exchanged with polyvalent metal ions. However, this zeolite is unfortunately applicable only to mercaptans.
A zeolite containing silver (Ag) is known to be effective in removing mercaptan-based, thiophene-based, and sulfide-based compounds. However, a zeolite containing silver is very expensive and its absorption selectivity for each sulfur component in a fuel gas containing various sulfur compounds is not known.
The kind, density, and component ratios of sulfur compounds included in a fuel such as city gas may differ according to the country, region, season, and time. Thus, when a single absorber is used in a desulfurization process, the performance and durability of the absorber with regard to each sulfur compound that might be contained in the fuel cannot be predicted. Therefore, a sulfur component may escape absorption and flow into a fuel cell stack due to an inaccurate prediction of the sulfur compounds that will be contained in a fuel and an inaccurate selection of the absorber.